Yup, Kootniz is correct about this for Grenadiers not being able to take the priests...
Howecer, having Grenadiers can allow the IG players to be able to field cheapest troops units for fulfilling the basic requirement in FOC. This would still allow the possibility to field other units in the Elite choices with the priest as you do not have to allocate them into the command squads. (Except the HQ command squad which requires one priest to be fielded in order to get the other priests into the units you want)
Note that it is still a good idea to have at least 1 infantry platoon for getting the infantry support. Many IG armies work best & are competitive with the basic infantry being the backbone of the army. So filling the basic requirement with only Grenadiers would not really help the army to be competitive. You could still do that, but it would require some good tactics & precise decision to allow the army being effective... (Basically, it is harder to play with only Grenadiers as the troop choice in the army)